1. Field of the Invention
The present invention relates to a method for forming an insulating film and a semiconductor device and, more particularly, a method for forming an insulating film used as an interlayer insulating film of a high integration density semiconductor integrated circuit device and having a low relative dielectric constant and a semiconductor device employing the insulating film therein.
2. Description of the Prior Art
In recent, reconsideration of interlayer insulating films for use in multilayered interconnections has been effected from a view point of high integration density or high speed operation of semiconductor integrated circuit devices.
More particularly, from view points of stability, easiness of film forming, etc., an SiO.sub.2 film has been widely used as an interlayer insulating film of the semiconductor integrated circuit device. But the problem of increase in parasitic capacitance has arisen according to progress of recent miniaturization of the circuit.
Conventionally, a low temperature film forming method has been used in forming the interlayer insulating film when taking influence on aluminum interconnection layers, etc. into consideration. For instance, the methods for forming the SiO.sub.2 film employing a plasma chemical vapor deposition method (referred to as PCVD method hereinafter) using tetraethylorthosilicate (referred to as TEOS hereinafter), i.e., (C.sub.2 H.sub.5 O).sub.4 Si have been proposed (see Patent Application Publication (KOKAI) 6-240459 and Patent Application Publication (KOKAI) 6-140386).
In the former (Patent Application Publication (KOKAI) 6-240459), when the SiO.sub.2 film is formed in the course of manufacturing a polysilicon thin film transistor (poly-SiTFT) which is used in view finder being requested to have a large area over 300 mm, CCD, liquid crystal projector and the like, the ratio of the flow rate of O.sub.2 gas or mixed gas of O.sub.2 and He to TEOS is set at more than 50 times. Impurities cannot stay even in the center of the large area substrate, and can therefore be removed from a surface of a substrate in the course of manufacturing to thus result in uniform film quality.
However, in the foregoing forming method of the SiO.sub.2 film, step coverage of the interlayer insulating film is lowered because of wiring steps. As a result, the foregoing forming method of the SiO.sub.2 film is not suitable for forming the interlayer insulating film of monolithic semiconductor integrated circuit such as memory, logic circuit, and the like.
In the latter (Patent Application Publication (KOKAI) 6-140386), it has been intended to reduce humidity absorption which causes degradation of device characteristics. The ratio of the flow rate of O.sub.2 to TEOS has been set at about 2.2 times, i.e., the flow rate of O.sub.2 has been set at 730 cc/min in contrast to the flow rate of TEOS of 330 cc/min.
However, since the SiO.sub.2 film formed by the PCVD method has high relative dielectric constant such as about 4.1, parasitic capacitance between interconnection layers is relatively large. When distances between interconnection layers are narrowed according to the progress in miniaturization of the circuit, parasitic capacitance has been further increased. This parasitic capacitance causes signal propagation delay, so an operation speed cannot be improved not to respond to miniaturization of the devices.
In other words, if relative dielectric constant of the interlayer insulating film is k and resistance of the interconnection layer is R, wiring delay time .tau. caused by the interconnection layers is in proportion to kR. Accordingly, it would be understood that material having small relative dielectric constant may be used as the interlayer insulating film to improve the wiring delay time. Parasitic capacitance can be reduced according to reduction in relative dielectric constant to thus decrease signal propagation delay.
Recently, as a method for forming an insulating film with low relative dielectric constant, several methods for forming the SiO.sub.2 film including fluorine, i.e., SiOF film by adding fluorine contained gas into material gas in terms of the PCVD method have been reported (see Fukada, Akahori, Extended Abstracts of the 1993 International Conference on Solid State Devices and Materials, Makuhari, 1993, pp.158-160; Usami, Shimokawa, Yoshimura, Extended Abstracts of the same, pp.161-163; and Mizuno, Hara et al., Extended Abstracts of the same, pp.510-512). The SiOF film is worthwhile to be used as interlayer insulating film of the next generation semiconductor integrated circuit device since its relative dielectric constant is lower than 4.1 in the conventional SiO.sub.2.
However, the SiOF film has unstable relative dielectric constant because of its high humidity absorption, like the SiO.sub.2 film formed by the PCVD method. In other words, if the SiOF film having low relative dielectric constant can be formed at first, the relative dielectric constant is increased as time elapsed. This is because water component absorbed into the SiOF film has significantly high relative dielectric constant like 80. Such water absorption into the SiOF film has resulted in not only increase in the relative dielectric constant but also bad influence on the semiconductor device per se, so that reliability of the semiconductor device is lessened.
Currently, as the insulating film having lower relative dielectric constant than the above insulating film, the insulating film formed of fluorine macromolecule has been known in the art. For example, relative dielectric constant of polytetrafluoroethylene (P-TFE) is 2.2 (1 MHz). Since the value is smallest in organic materials, P-TFE is promising as the insulating film.
However, since fluorine macromolecule is in general insoluble into most solvents, they cannot be treated by spin coating or press coating as in ordinary photosensitive resins. So they have such drawbacks that it is difficult to form thin films by fluorine macromolecule.
In addition, although some of fluorine macromolecule materials which can be treated by spin coating are in market as a merchandise, they have poor adhesiveness and heat resistance. As a result, they have such drawbacks that decomposition or exfoliation from the substrate occurs in heating the substrate.
Besides, to overcome the problem of such adhesiveness, there are some reports wherein P-TFE thin film has been formed by plasma polymerization. However, the plasma polymerization film can provide good adhesiveness, but it has the drawback that heat resistance is poor. Furthermore, since polymer of low molecular weight is included into the film and released from the film when the substrate is heated, merely the film having relative dielectric constant such as about 2.7 (1 MHz) can be obtained.
For example, the following method has been proposed by Kazuhiko Endo and Toru Tatsumi. That is, using the parallel plate type plasma CVD equipment (CCP=Capacitive Coupled Plasma) and the helicon wave plasma CVD equipment, amorphous carbon fluoride film (fluorine type resin film) has been formed under the conditions that CF.sub.4 +CH.sub.4 or C.sub.2 F.sub.6 +CH.sub.4 is used as material gas and substrate temperature is at 50.degree. C. (see Material Research Society, symposium proceedings, Vol.381, entitled Low-Dielectric Constant Materials-Synthesis and Applications in Microelectronics, 1995, pp.249 to 254).
According to this method, the fluorine type resin film enabling excellent heat resistance would be obtained by mixing CH.sub.4 as the hydrogen containing compound into the film when forming the film. More specifically, in stacking the film, fluorine (F) radical serving as the basis of etching operation is gettered by hydrogen (H) to reduce its etching operation and at the same time to produce carbon rich state so as to enhance bridging density. The fluorine type resin film having relative dielectric constant of 2.1 could be obtained by the parallel plate type plasma CVD equipment, and also the fluorine type resin film having relative dielectric constant of 2.4 could be obtained by the helicon wave plasma CVD equipment.
However, there has been caused the problem that the relative dielectric constant is increased to 2.7 by annealing process at 300.degree. C.
According to the film forming method excluding the film forming method using the helicon wave plasma CVD equipment and employing C.sub.2 F.sub.6 +CH.sub.4 as material gas, there has been caused the problem that residual film rate is lessened less than 70% by annealing process at 300.degree. C. for one hour to render heat resistance worse.
On the other hand, according to the film forming method using the helicon wave plasma CVD equipment and employing C.sub.2 F.sub.6 +CH.sub.4 as material gas, residual film rate becomes 100% after annealing process at 300.degree. C. for one hour, but residual film rate is lessened less than 60% by annealing process at 400.degree. C.
Similarly, the following method has also been proposed by Kazuhiko Endo and Toru Tatsumi. That is, using the parallel plate type plasma CVD equipment and employing CF.sub.4 +CH.sub.4 added by N.sub.2 as material gas, amorphous carbon fluoride film (fluorine type resin film) has been formed at substrate temperature of 50.degree. C. (see Extended Abstracts of the 1995 International Conference on Solid State Devices and Materials, Osaka, 1995, pp.177 to 179).
In this case, fluorine type resin film having relative dielectric constant of 2.5 could be obtained. The relative dielectric constant remains at 2.5 and is scarcely changed after annealing process at 300.degree. C. for one hour, but there have been drawbacks that residual film rate is lessened less than 90% and in addition heat resistance becomes poor.
In two foregoing reports as for the amorphous carbon fluoride film (fluorine type resin film), annealing temperature denotes heater temperature. It may thus be supposed that actual temperature of the amorphous carbon fluoride film would be considerably lower than the heater temperature.
In the meanwhile, it can be supposed that characteristics which are required for the interlayer insulating film in the high integration density semiconductor device to be formed using a 0.18 .mu.m design rule are that the relative dielectric constant is less than 2.5 and residual film rate is almost 100% after heat treatment at more than 300.degree. C.
However, these characteristics are difficult to be satisfied by the amorphous carbon fluoride film formed by the above two film forming methods.